1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting a defect such as a crack, a recess, a cavity, or the like in a test object.
2. Prior Art
Cylinder/piston mechanisms for use in automotive engines may fail to operate properly if cylinders and pistons have defects such as cracks, cavities, recesses, or the like. Parts with such cracks, cavities, or recesses should preferably be sorted out on parts production lines before they are assembled into engines.
Various nondestructive testing methods are known for detecting these defects in engine components. For example, engine component deficiencies have been detected by an ultrasonic echo method, an acoustic emission method which detects a sonic energy wave caused when a crack is produced in a component, a CCD camera image observation method, a radiographic method, a visual-optical method using a color check, and an eddy current method, among others.
However, the above testing methods have been problematic for the following reasons:
The ultrasonic echo method uses a transducer or sensor for transmitting and receiving ultrasonic energy into and from a test object. In use, the sensor is held in contact with the test object. Since, however, the ultrasonic energy is propagated linearly through the test object, only the area of the test object which is in contact with the sensor can be tested. The waveform of the ultrasonic energy received from the test object tends to vary due to reflections caused by inadequate coupling between the sensor and the test object or due to slight changes in the angle of the sensor with respect to the test object. Consequently, it is not easy to achieve proper determination of defects in test objects.
The acoustic emission method also employs a sensor held in contact with a test object. Inasmuch as a crack is detected on the basis of an acoustic emission signal produced when the crack is caused, a crack can be detected only while the crack is being developed. Difficulty arises in this testing method unless the crack to be detected is increasing in a test object.
The CCD camera image observation method is liable to detect discolored spots and patterns other than real defects and is not effective to detect cavities known as blowholes in castings.
The radiographic method allows the operator to make a visual inspection of the internal structure of a test object. However, it is cumbersome to regulate the dose of X-rays to be applied to a test object. A crack in a test object may not be observed if the dose of applied X-rays is not properly regulated. Since the radiographic method cannot efficiently inspect all test objects that need to be tested, it is not suitable for inspection on production lines.
In the eddy-current method, a test object is rotated at high speed, and a sensor has to be positioned closely to and uniformly moved with respect to the test object. If the test object has surface irregularities, however, measurements are difficult to achieve because the sensor cannot be positioned near the test object.